The implementation now distinguishes a newly placed image cell from one later overwritten by text. On a SIXEL terminal, damaged cells are omitted from image output; on Kitty they remain ordinary positional placements.

grid.c

@@ -778,6 +778,10 @@ grid_set_cells(struct grid *gd, u_int px, u_int py, const struct grid_cell *gc,
 	struct grid_line	*gl;
 	struct grid_cell_entry	*gce;
 	struct grid_extd_entry	*gee;
+	const struct grid_cell	*new_gc;
+#ifdef ENABLE_IMAGES
+	struct grid_cell	 old_gc, image_gc;
+#endif
 	u_int			 i;
 
 	if (grid_check_y(gd, __func__, py) != 0)
@@ -791,11 +795,25 @@ grid_set_cells(struct grid *gd, u_int px, u_int py, const struct grid_cell *gc,
 
 	for (i = 0; i < slen; i++) {
 		gce = &gl->celldata[px + i];
+		new_gc = gc;
 #ifdef ENABLE_IMAGES
 		{
 			u_int old_id = grid_entry_image(gl, gce);
-			u_int new_id = (gc->flags & GRID_FLAG_IMAGE) ?
-			    gc->image_id : 0;
+			u_int new_id;
+
+			if (old_id != 0 && (~gc->flags & GRID_FLAG_IMAGE)) {
+				grid_get_cell1(gl, px + i, &old_gc);
+				memcpy(&image_gc, gc, sizeof image_gc);
+				image_gc.flags |= GRID_FLAG_IMAGE|GRID_FLAG_IMAGE_DAMAGED;
+				image_gc.image_id = old_gc.image_id;
+				image_gc.image_x = old_gc.image_x;
+				image_gc.image_y = old_gc.image_y;
+				new_gc = &image_gc;
+			}
+			if (new_gc->flags & GRID_FLAG_IMAGE)
+				new_id = new_gc->image_id;
+			else
+				new_id = 0;
 
 			if (old_id != new_id) {
 				if (old_id != 0)
@@ -805,11 +823,11 @@ grid_set_cells(struct grid *gd, u_int px, u_int py, const struct grid_cell *gc,
 			}
 		}
 #endif
-		if (grid_need_extended_cell(gce, gc)) {
-			gee = grid_extended_cell(gl, gce, gc);
+		if (grid_need_extended_cell(gce, new_gc)) {
+			gee = grid_extended_cell(gl, gce, new_gc);
 			gee->data = utf8_build_one(s[i]);
 		} else
-			grid_store_cell(gce, gc, s[i]);
+			grid_store_cell(gce, new_gc, s[i]);
 	}
 }
 
@@ -1322,7 +1340,7 @@ grid_string_cells(struct grid *gd, u_int px, u_int py, u_int nx,
 #ifdef ENABLE_IMAGES
 		if (gc.flags & GRID_FLAG_IMAGE) {
 			utf8_set(&gc.data, ' ');
-			gc.flags &= ~GRID_FLAG_IMAGE;
+			gc.flags &= ~(GRID_FLAG_IMAGE|GRID_FLAG_IMAGE_DAMAGED);
 			gc.image_id = gc.image_x = gc.image_y = 0;
 		}
 #endif

image.c

@@ -95,7 +95,8 @@ static const struct image_backend image_backend_fallback = {
 	"fallback", IMAGE_BACKEND_SCROLLS, NULL, NULL, NULL
 };
 static const struct image_backend image_backend_sixel = {
-	"sixel", IMAGE_BACKEND_GRAPHICAL, sixel_draw_rect,
+	"sixel", IMAGE_BACKEND_GRAPHICAL|IMAGE_BACKEND_TEMPORAL,
+	sixel_draw_rect,
 	sixel_free_output, sixel_geometry_changed
 };
 
@@ -532,9 +533,15 @@ image_get_draw_cell(struct tty *tty, const struct grid_cell *gc,
 
 	if (image_backend_flags(tty) & IMAGE_BACKEND_GRAPHICAL) {
 		memcpy(out, gc, sizeof *out);
-		out->flags &= ~(GRID_FLAG_IMAGE|GRID_FLAG_SELECTED);
+		out->flags &= ~(GRID_FLAG_IMAGE|GRID_FLAG_IMAGE_DAMAGED|
+		    GRID_FLAG_SELECTED);
 		return (1);
 	}
+	if (gc->flags & GRID_FLAG_IMAGE_DAMAGED) {
+		memcpy(out, gc, sizeof *out);
+		out->flags &= ~(GRID_FLAG_IMAGE|GRID_FLAG_IMAGE_DAMAGED);
+		return (0);
+	}
 	image_get_fallback_cell(tty, im, gc->image_x, gc->image_y, gc, out,
 	    style_ctx);
 	return (0);
@@ -545,6 +552,7 @@ void
 image_set_cell(struct grid_cell *gc, struct image *im, u_int x, u_int y)
 {
 	/* Keep the cell contents as the underlay for transparent pixels. */
+	gc->flags &= ~GRID_FLAG_IMAGE_DAMAGED;
 	gc->flags |= GRID_FLAG_IMAGE;
 	gc->image_id = im->id;
 	gc->image_x = x;
@@ -677,6 +685,11 @@ image_draw_line(struct tty *tty, struct screen *s, u_int px, u_int py,
 			run = 1;
 			continue;
 		}
+		if ((backend->flags & IMAGE_BACKEND_TEMPORAL) &&
+		    (gc.flags & GRID_FLAG_IMAGE_DAMAGED)) {
+			run = 1;
+			continue;
+		}
 		im = image_find(gc.image_id);
 		if (im == NULL) {
 			run = 1;
@@ -685,6 +698,8 @@ image_draw_line(struct tty *tty, struct screen *s, u_int px, u_int py,
 		for (run = 1; i + run < nx; run++) {
 			grid_view_get_cell(s->grid, px + i + run, py, &next);
 			if (~next.flags & GRID_FLAG_IMAGE ||
+			    ((backend->flags & IMAGE_BACKEND_TEMPORAL) &&
+			    (next.flags & GRID_FLAG_IMAGE_DAMAGED)) ||
 			    next.image_id != gc.image_id ||
 			    next.image_y != gc.image_y ||
 			    next.image_x != gc.image_x + run)

screen-write.c

@@ -2607,6 +2607,9 @@ screen_write_cell(struct screen_write_ctx *ctx, const struct grid_cell *gc)
 	struct grid_line	*gl;
 	struct grid_cell_entry	*gce;
 	struct grid_cell	 tmp_gc, now_gc;
+#ifdef ENABLE_IMAGES
+	struct grid_cell	 image_gc;
+#endif
 	struct tty_ctx		 ttyctx;
 	u_int			 sx = screen_size_x(s), sy = screen_size_y(s);
 	u_int			 width = ud->width, xx, not_wrap, i, n, vis;
@@ -2651,6 +2654,20 @@ screen_write_cell(struct screen_write_ctx *ctx, const struct grid_cell *gc)
 		return;
 	screen_write_initctx(ctx, &ttyctx, 0, 0);
 
+#ifdef ENABLE_IMAGES
+	/* Update the text underlay without removing an image placement. */
+	grid_view_get_cell(gd, s->cx, s->cy, &now_gc);
+	if ((now_gc.flags & GRID_FLAG_IMAGE) &&
+	    (~gc->flags & GRID_FLAG_IMAGE)) {
+		image_redraw_area(ctx, s->cx, s->cy, width, 1);
+		memcpy(&image_gc, gc, sizeof image_gc);
+		image_gc.flags |= GRID_FLAG_IMAGE|GRID_FLAG_IMAGE_DAMAGED;
+		image_gc.image_id = now_gc.image_id;
+		image_gc.image_x = now_gc.image_x;
+		image_gc.image_y = now_gc.image_y;
+		gc = &image_gc;
+	}
+#endif
 	/* Handle overwriting of UTF-8 characters. */
 	gl = grid_get_line(s->grid, s->grid->hsize + s->cy);
 	if (gl->flags & GRID_LINE_EXTENDED) {

tmux.h

@@ -810,6 +810,7 @@ struct colour_palette {
 #define GRID_FLAG_CLEARED 0x40
 #define GRID_FLAG_TAB 0x80
 #define GRID_FLAG_IMAGE 0x100
+#define GRID_FLAG_IMAGE_DAMAGED 0x200
 
 /* Grid line flags. */
 #define GRID_LINE_WRAPPED 0x1
@@ -4240,6 +4241,7 @@ char		*regsub(const char *, const char *, const char *, int);
 /* image.c */
 #define IMAGE_BACKEND_GRAPHICAL 0x1
 #define IMAGE_BACKEND_SCROLLS   0x2
+#define IMAGE_BACKEND_TEMPORAL  0x4
 
 struct image	*image_create(u_int, u_int, u_int, u_int, u_int, u_int,
 		     u_char *);